IPhone hasn't shown any TT mail since Monday am - works OK on other devices and with other accounts

I have not received any TalkTalk mail on my iPhone 6 since Sunday evening. All the old mails have disappeared. The phone works fine on other e-mail accounts (eg BTconnect and iCloud). I can access my TT mail on my iPad or my desktop. The phone says there is something wrong with my settings but I have not changed these and have checked them and they are as recommended.